Do foraging bumblebees scent-mark food sources and does it matter?

The foraging of worker bees of Bombus terrestris visiting artificial feeders in a climatic test chamber was investigated. The behaviour of worker bees visiting rewarding and unrewarding feeders is completely different. Of all flower visits to rewarding feeders 94% are probing-visits, i.e. the bees land on the flower and probe for nectar. In contrast, only 0.3% of all visits to unrewarding feeders are probing-visits, whereas 47% are approach-visits, i.e., the bees approach the feeders without landing. Exchanging feeder discs proves that the signal used for discrimination must be associated with the plastic disc used as landing platform. Most probably it involves scent-marking of the rewarding feeders with components of high and low volatility. The mean foraging efficiency of bees in a scent-marked foraging arena is 5.7 mg sugar/min and drops to 2.8 mg sugar/min after the scent marked discs are replaced by clean ones. Three components generate this drop in foraging efficiency: (1) the between-flower flight time increases, i.e. the bees search for a longer time before landing on flowers, (2) the bees no longer discriminate between rewarding and unrewarding feeders, and (3) the bees probe empty feeders longer than necessary; obviously they expect to find nectar.

Floral color change and the attraction of insect pollinators in lungwort (Pulmonaria collina).

We studied the effect of floral color change on long- and short-distance attraction of insect pollinators to the herb lungwort, Pulmonaria collina. Lungwort flowers change color with age from red to blue. Young red flowers had a significantly greater pollen and nectar reward and were significantly more often unpollinated than old blue ones. Red and blue flowers both influenced long-distance attractiveness of plants, defined as the number of insect approaches towards an individual plant. After reaching a plant, flower visitors preferred to visit young red flowers. Therefore, short-distance attractiveness, defined as the number of flowers visited successively on an individual plant, was influenced mainly by the number of young red flowers. The co-occurrence of the change in reproductive ability, in amount of reward, and in flower color enabled lungwort plants to direct pollinators to reproductive, highly rewarding red flowers. The data suggest that by maintaining changed flowers lungwort plants can increase their long-distance attraction and simultaneously enhance the probability of flower visits to pre-changed flowers. Thus, we propose floral color change as a mechanism that can increase the efficiency of pollen transfer to enhance plant fitness.